It's a familiar scene if you've ever watched a hospital show. The strapped-down patient is pushed into the narrow tube of a functional magnetic resonance imaging (fMRI) machine and within minutes doctors scan the inner workings of the brain as they search for tumors or lesions. But could the brain scan perhaps also reveal whether the patient likes a new Nokia (NOK) phone or thrills to the vroom of a Harley-Davidson (HOG)? That's the premise of neuromarketing. It's a hot trend and the subject of Buyology: Truth and Lies About Why We Buy by Danish consultant Martin Lindstrom.

Lindstrom tells us early on that his neuromarketing research took three years, spanned much of the globe, and cost millions of dollars. (Eight unnamed corporations picked up the tab.) But the research takes up only a fraction of this uneven and sometimes frustrating book. For many more of its pages, we accompany Lindstrom on a broad tour of marketing. He discusses the sexual appeal of Abercrombie & Fitch's (ANF) pitch, the history of subliminal ads, and the origins of the lime slice in a bottle of Mexican beer. Lindstrom entices us with the research, which seems to explain much about human behavior, but delivers it too sparingly. "Before we get to our fMRI test and its startling results," he writes at one point, "let's do a little mind experiment of our own."

Yet another detour. Still, the research in Buyology points to a vital trend: Marketers are feasting on new streams of customer data. They can track our wanderings on the Internet and our purchases at the supermarket, and they can start to predict an individual's behavior. The brain scans—both an advanced version of the electroencephalograph, which employs wired skullcaps, and the more expensive fMRI—provide rich new data streams. But drawing definitive conclusions from them is not easy.

Scientists have mapped out the regions of the brain that dominate lust, anger, attention, our protective instincts, and much more. But when those regions light up with activity, it's not clear what behavior will follow—or how it may vary from one person to the next. Scientists' understanding of such brain activity is like the early cartographers' grasp of geography.

In one of his most interesting chapters, Lindstrom delves into the value of product placement—the branded phones, laptops, and liquors that share the screen for a precious second or two with movie or TV stars. Such positioning is especially important now that viewers have tools to zap traditional 30-second ads. But does product placement work? Lindstrom's researchers place brain-monitoring caps on viewers watching American Idol and then study their responses to the Cokes the judges sip, the Fords the contestants pile into between acts, and Cingular, the cell-phone service that lets the public vote.

As it turns out, the viewers emerge with far stronger memories of Coke than of Cingular (T). Ford scores worst of all. Lindstrom theorizes that Coke fares better because it insinuates itself into the action. The judges interact with it while they deliberate. Cingular, by contrast, is just a tool for the TV viewers, and Ford is a mere advertisement. In the end, the carmaker doesn't just lose: Its message is annihilated. Viewers seem to remember less about the company after seeing the commercials than before. Lindstrom goes so far as to suggest that the Coke placements push Ford out of people's memories: The automaker spent $26 million in yearly sponsorship only to lose mindshare.

Like much of Buyology, this conclusion is open to debate. Perhaps good feelings about Ford reside deep in viewers' subconscious and will surface months later. Who's to say? Even with the latest technology, much of the activity in the brain remains invisible, or indecipherable, to us.

Take my brain. As I read the last two chapters of Buyology on a train, a chatty couple sits next to me. They talk. Try as I might, I can't help but listen. If I were wearing a wired skullcap, the patterns might suggest that the duo has captured my attention. Does it mean I'm a fan of theirs? Not by any stretch. Is another region in my brain signaling anger or frustration? Is it shining bright? It's tough to translate these brain patterns. But neuromarketers, including Lindstrom, are busy working at it.